Cellular and organ level localization of maltose in maltose-excess Arabidopsis mutants.

Maltose is the predominant form of carbon exported from the chloroplast at night. Plants that lack either the chloroplast maltose transporter or disproportionating enzyme 2 (DPE2, EC 2.4.1.25) have excess maltose in leaves. We confirmed that DPE2 is not associated with the chloroplast in Arabidopsis thaliana. Using non-aqueous fractionation methods, we found that essentially all the maltose in mex1-1 leaves is located inside chloroplasts but only 40% of maltose in dpe2-1 leaves is located inside chloroplasts. We found that maltose exists in a significant amount in the exudates collected from maltose-accumulating dpe2-1 Arabidopsis petioles. However, the amount of maltose in the exudates from mex1-1 petioles was not significantly different from that in wild-type phloem exudates. We found twice as much maltose in the roots of dpe2-1 plants relative to wild type but the maltose level in the roots of mex1-1 plants was not higher than wild type. We conclude that maltose accumulated in the cytosol of leaves can be carried from the shoots to the roots and that maltose accumulated in the chloroplast of mex1-1 leaves is not mobilized. By measuring the transcript levels and enzymatic activities, we show that maltose-metabolizing enzymes are active in wild-type roots. The amount of maltose moved from the shoots to the roots increased in dpe2-1 plants. The roots of dpe2-1 plants must have the capacity to metabolize the excess maltose.